Both economic growth and population growth have driven the ever-increasing demand for energy and water. Water scarcity already affects every continent. Almost one-fifth of the world's population lives in areas of physical scarcity and an increasing number of regions are chronically short of water. A substantial amount of fresh water is wasted, polluted, and unsustainably managed. Less than one percent of water on the earth's surface is suitable as an eligible water source for direct consumption in domestic or industrial applications. More energy-intensive water is currently being used to compensate for the decline in water availability. Purification and desalination of wastewater, seawater, or brackish water become an important, but expensive, solution for water supply issues. While distillations to potable water are needed for human survival, there are also huge demands for other distillation applications, such as irrigation in agriculture or planation and water-assisted industrial processes, which may tolerate less purification requirements including allowance with condensable vapors other than water vapor.
Distillation involves the production of evaporative/condensable solvents from non-evaporative solvents and/or solutes, such as fresh water from waste water or saline water. Distillation is one of the earliest forms of water treatment. There are two basic methods: thermal distillation and membrane separation. Thermal distillation uses heat to boil to-be-treated water into vapor, leaving the non-evaporative solvents or solutes behind, that is collected and condensed back into water by cooling it down. Some derivatives of the methods include vacuum distillation processes used in refineries. The process involves vaporous extraction of water. Vapor compression distillation (VCD) is a thermal desalination process, such as MSF (multi-stage flash) and MED (multiple effect distillation), which has in the past provided the majority of potable water in regions where excess heat from power plants is used to heat and desalinate seawater. These are efficient and viable solutions when there is waste heat or sufficient electricity available.
In membrane separation, seawater is forced through a semipermeable membrane that separates salt from water. The most common type of membrane separation is reverse osmosis. Membrane distillation (MD) uses hydrophobic membranes and differences in vapor pressure for separation and distillation. The vapor pressure difference across a membrane can be generated by pressurization (such as in reverse osmosis distillation), heating (such as in thermal membrane distillation), or vacuuming, or their combination. Common configurations of membrane distillation are direct contact MD, air gap MD, and vacuum MD. Vapor compressor has been used in some MD processes to increase the difference in vapor pressure in membrane distillation. Vacuum has also been applied in other MD processes to maintain the pressure in the permeate side of the membrane at less than the saturation pressure of the vapor to be separated from the hot feed solution.
Vacuum membrane distillation (VMD) uses vacuum to achieve higher partial pressure gradients, and hence higher permeate flux. The use of vapor compressor or the conventional vacuum pump contributes to the overall energy consumption of these MD processes. The use of hydrophobic membranes in MD and VMD distillation processes incurs operational cost due to the need for cleaning and replacement of membranes, in addition to the other limitations in applications such as hydrophobicity requirement between solutions and membrane and operation limits in temperature and pressure of membrane materials.
Multi-stage flash distillation (MSF) can be either a once-through (no brine recirculation) or brine recirculation flow system. Successive evaporation of brine water into flash steam is coupled with condensation inside the stages, such that the evaporation latent heat is recovered by preheating feed seawater. During the process, the feed flows through cooling tubes inside the condensation chambers of stages to receive the latent heat of the vapor to produce condensate and vacuum. The direction of the flow is from the last stage to the first stage and the temperature of the feed is increased by each stage. After exiting the first stage, the temperature of the feed is further increased by a heater. The heated feed is then flows through the evaporation chambers of stages, from the first to the last. The vacuum formed in the condensation chamber of the stage reduces the pressure inside the evaporation chamber below the vapor pressure of the liquid, inducing the liquid inside the chamber to evaporate at a rate self-sustained with the condensation later in the condensation chamber. An orifice is placed on the flow path of the feed between two neighboring stages for reducing the pressure of the flow. Demisters are placed between the evaporation and condensation chambers to remove the entrained brine droplets from the vapor. This is essential to prevent increase in the salinity of product water or scale formation on the outer surface of the condenser tubes.
Seawater has many different gases dissolved in it, especially nitrogen, oxygen, carbon dioxide and argon. They do not react chemically and are not easily condensed by cooling. They are referred to as non-condensable gases (NCG). The presence of non-condensable gases may also be caused by the leakage of ambient air into the process operating under vacuum. The presence of non-condensable gases is a serious problem in seawater distillation. Extensive pretreatment, which includes de-aeration, antifoam, and anti-scalent additions, is often applied to the feed stream, in addition to the removal of suspended solids.
Thus there still remains a need in the art for a system and method to avoid the above issues. In addition, there remains a need for a distillation and desalination system that prevents the accumulation of non-condensable gases and avoids surface scaling or fouling associated with current systems, as well as improves efficiency of thermal energy utilization.